The following discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was published, known or part of the common general knowledge in any jurisdiction as at the priority date of the application.
Sludge is a semi-solid slurry which includes but is not limited to sewage sludge, biogas residues, paper sludge, and food and beverage sludge. Sludge variants can carry heavy metals, toxins, pollutants and pathogens. Therefore, if sludge is not properly treated before disposal, there will be a risk of disease spread, heavy metal poisoning and environmental damage. Treatment of sewage or wastewater sludge generated from wastewater treatment plants typically comprises the following steps: thickening, dewatering and drying. Drying by heat removes the sludge's moisture content and can destroy pathogens and neutralise toxins. The removal of moisture and application of heat to dry the sludge can result in safe and easy handling of the dried sludge for downstream treatment/disposal.
Several methods have been developed to dry sludge and these include but are not limited to convection, radiation and indirect (contact or conduction) drying methods. In convection methods, heated dry air is brought into contact with the sludge in a drum or belt dryer; in radiation methods, heat radiated by heating elements is used to dry the sludge, where such heat can come from solar radiation or infrared heating elements; and in indirect methods, the sludge is brought into contact with a surface heated by a heat source for drying.
U.S. Pat. No. 5,091,079 discloses an apparatus which uses two ovens to provide induction heating to reduce sludge especially those sludge containing heavy metal and a vacuum evacuation chamber to draw gases and vapours from the sludge being reduced, where the sludge travels through the apparatus along a single conveyor belt.
KR Patent Registration No. 10-1005086 and KR Patent Registration No. 10-0976243 disclose a sludge drying apparatus which uses a heated roller or drum to dry sludge. Thermal oil is used as a heating medium where it is pumped to the interior of the roller, and conducts heat to the roller's interior circumferential surface. The conducted heat then travels to the exterior circumferential surface of the roller, against which the sludge being dried is compressed via a belt. There are several disadvantages associated with these drying apparatuses. Firstly, heat is conducted from the side of the sludge in contact with the roller to the other side, where moisture and evaporated vapour has to travel across the thickness of the sludge in order to escape, since the roller will likely have an impenetrable/non-porous surface to prevent the thermal oil from leaking out. Therefore thickness of sludge being treated is limited due to the way heat is conducted only at one side of the sludge and the relatively long escape route of the moisture and vapour from one side of the sludge to the other. Secondly, the thermal oil has to be continuously recirculated from the rollers to boilers/heaters to replenish any heat loss. During such recirculation, there is inherent heat loss along the thermal oil piping, boiler/heater and portions of the rollers which are not used to dry the sludge, thereby leading to energy inefficiencies.
Thirdly, boilers/heaters, thermal piping and large pumps to circulate the oil will result in a large footprint for the entire apparatus. Fourthly, for single or multiple rollers configurations, the thermal oil is typically drawn from the same source, i.e. a single boiler. Therefore, the drying temperature cannot be accurately controlled and varied at different drying stages. Fifthly, heating temperature is limited by the maximum thermal oil operating temperature. Finally, a substantial amount of time (approximately one hour) is required to warm the apparatuses from room temperature to the desired drying temperature. Furthermore, as it will be unsafe to conduct maintenance and repair works while the apparatuses are still hot, a substantial amount of time is required to fully cool the apparatuses down—typically more than 3 hours to cool down and reach 100° C.
Therefore there is a need to alleviate problems in the prior art, such as to increase the efficiencies of the drying process of the prior art apparatuses, further reduce the moisture content of treated sludge, improve the quality of end products, decrease the foot print of the prior apparatuses, and improve the time required for heating and cooling down.